Rescued Lab Mix/Intro

Hi! New to the Forum. I acquired what is allegedly a Black Lab/Neopolitan Mastiff mix from a complete idiot who was in the process of "dumpstering" him at 6 weeks as the pup was the "runt" of the litter. Bottle fed him until he graduated to puppy chow. He's now 6 months old, had all his shots, neutered. Stands 18" at the shoulder, weighs 61 lbs, and is healthy as a horse, strong as an ox! He self-trained himself to go outside to do his business. Very well mannered pup, has the basic commands down pat (sit, stay, lay down, come). I do have one question, however, having owned a VERY large Dane/Mastiff previously.....Bo, at 6 months now is 18" at the shoulder, 61 lbs, and I believe this to be a weee bit bigger than a Lab/Neopolitan Mastiff should be at this age..Neo's aren't nearly as Large as Bull Mastiffs, and I'm beginning to wonder if Bo is mixed with Dane or Bull Mastiff, and not a Neo at all. Without a DNA test (pricey!), I'm hoping someone on the Forum can guess just WHAT mix he may be. I have no experience with Lab mixes, owned mostly Chows throughout my life. Any ideas?

a picture from the front might be more use but I just did a quick google andthere doesn't seem to be much difference height wise between a Neo and a Bull Mastiff but that aside 18 inches to the shoulder doesn't seem big at all. My male Dobermann was 27 inches at the shoulder fully grown but reached that height around 8 months, my female was slightly shorter at 25 inches but again was full height relatively early.
Just enjoy him for what he is, he sounds like a great dog.

Thanks....perhaps I am imagining things, he just seem a bit large for 6 months. My Dane/Mastiff when full grown was 29-30", and had a 30" neck, weighed around 165-170, but I don't remember how big he was at 6 months.